Magnetron having rapid starting property when cold



SeptQ24, 1968 F. J. WEAVER 3,403,281

MAGNETRON HAVING RAPID STARTING PROPERTY WHEN COLD Filed June 10, 1966 Zlgzm Amy:

United States Patent 3,403,281 MAGNETRON HAVING RAPID STARTING PROPERTY WHEN COLD Francis James Weaver, Essex, England, assignor to English Electric Valve Company Limited, London, England, a British company Filed June 10, 1966, Ser. No. 556,663 Claims priority, application Great Britain, Aug. 16, 1965, 34,961/ 65 7 Claims. (Cl. 313337) This invention relates to magnetrons and has for its object to provide improved high power magnetrons which will start up rapidly from cold.

As is well known, much of the cathode emission of a high powered magnetron is provided, when the magnetron is in normal operation, by secondary emission resulting from bombardment of the cathode by returning electrons of the electron cloud in the tube. Normal present day high power magnetron practice is to Provide a massive thermionic cathode having the usual built-in heater which is switched on when the magnetron is required to be brought into use and which heats the cathode to cause it to emit primary electrons. The cathode surface is suitably constructed so as to be of high secondary emission coefiicient and, some little time after switching on, electrons from the electron cloud in the tube bombard the cathode thus causing secondary emission which supplements the initial primary thermionic emission. This secondary emission then rapidly increases until, when the magnetron is in full normal operation, it very substantially exceeds the initial thermionic primary emission. It is common for the design of high power magnetrons to be such that when the normal operating condition is reached the cathode heater can be switched off.

Known magnetrons of the foregoing nature have the defect that the heat inertia of the cathode structure, which, in a high power magnetron is massive, is large, with the result that, when starting from cold, an appreciable length of time elapses, after switching on the cathode heater, before the cathode warms to the required temperature and sufiicient primary emission is produced. In many magnetron applications this is a serious defect.

According to this invention a mangetron is provided with a cathode structure comprising a secondary electron emissive cathode member adapted to provide at least the major part of the required electron emission by a secondary emission by electron bombardment when the magnetron is in normal operation, and a filamentary thermionic emitter adapted to provide emission for rapidly starting said magnetron into operation from cold.

Preferably the secondary emissive cathode member is an externally grooved body having an outer surface of high secondary electron emission coefficient and the filamentary thermionic emitter is constiuted by filament wire positioned in the grooving.

In one way of carrying out the invention the secondary emissive cathode member is helically grooved on its external surface and thermionic emissive filamentary wire is positioned in the helical grooving.

In another way of carrying out the invention the secondary emissive cathode member is formed with grooves distributed round it and running parallel to its axis and thermionic emissive filamentary wires are positioned in these grooves.

In one form of embodiment the secondary emissive cathode member is a massive solid body of beryllium alloy having an oxidised surface adapted to provide the required secondary emission at a relatively low temperature (below about 600 C.), said body being kept down to said low temperature, when the magnetron is in normal operation, with the aid of loss of heat by substantial heat conduction occurring through said body.

3,403,281 Patented Sept. 24, 1968 ice In another form of embodiment the secondary emissive cathode member is a barium aluminate impregnated sleeve adapted to provide the required primary and secondary emission at a temperature in the neighborhood of 950 to 1000 C. when the magnetron is in normal operation.

The filamentary thermionic emitter may be of any suitable material known per se e.g. tungsten wire.

The materials used in the above described secondary emissive cathode membersbaryllium alloy or barium -aluminatehave quite good electrical conductivity and in such cases it is therefore necessary to prevent the occurrence of short-circuiting electrical contact between the filament or filaments and the secondary emissive cathode member. Such prevention may be achieved in some constructions solely by tensioning the filament material to hold it in spaced relation with the adjacent surfaces of the said member. In some cases however insulating material must be provided for this purpose e.g. the filament wire may be threaded through spaced insulating beads which space it from said member at required intervals along its length and/ or the surfaces of the said cathode member adjacent the filament or filaments may be provided with a layer of insulation e.g. alumina sintered thereon.

The invention is illustrated in the accompanying drawings in which FIGURES 1 and 2 are mutually perpendicular views illustrating one embodiment and FIGURE 3 and 4 are mutually perpendicular views illustrating another embodiment. The drawings are to a large extent diagrammatic, no attempt having been made to show close detail. Except for the cathode structure a magnetron in accordance with this invention may be as known per se and accordingly, in the drawings, only cathode structures are shown.

Referring to FIGURES 1 and 2 the cathode structure therein shown comprises a directly heated filament 1 supported in and insulated from a helical groove 2 formed in a solid cylindrical body 3 having a surface of high secondary emission co-efiicient. The cathode body 3 may also be capable of significant thermionic primary emission and in practice probably will be, though this primary emission capability is not essential and will not, in normal practice, contribute to any very great extent to the total emission. The body 3 may be of a beryllium alloy such as beryllium copper with an oxydized surface which, when the magnetron is in normal operation, operates at a relatively low temperature i.e. a temperature not exceeding about 600 C. This low temperature operation is maintained largely by loss of heat 'by massive thermal conduction through the body 3 to a suitable heat conducting and radiating support structure. Such a structure appears at 4 in the sectioned part of FIGURE 1.

In the construction of FIGURES 1 and 2 the filament 1 follows the helical groove and at its ends passes through holes 5 to an axial bore 6 formed in the body 3. One of the connections to the filament 1 appears in FIGURES 1 at 7.

Beryllium copper is of good electrical conductivity so that it is necessary to insulate the filament from the body 3 so as to prevent its being short circuited by the body. Any convenient means (not shown) may be provided for this purpose. Thus for example, the filament 1 may be spaced off from the walls of the groove and of the holes 5 by small spaced insulating beads threaded on the filament. This arrangement is, however, not preferred in the construction of FIGURES 1 and 2 because, owing to the helical shape of the filament, the beads would have to be fairly closely spaced along the filament and this is an obvious disadvantage. It is therefore preferred in the construction of FIGURES 1 and 2 to insulate the filament from the body 3 by providing the latter with a layer of 3 V insulation suitably positioned on the walls of the groove and of the holes in the neighbourhood of the filament. Thus a layer of alumina insulation could be sintered on to the base of the groove and the walls of the holes 5 and the filament bedded on this. In FIGURE 1 the parts 8 are the normally provided nickel or molybdenum or similar end hats for localising the electron emission.

In the modification shown in FIGURES 3 and 4 the secondary emissive part of the structure is provided by a barium aluminate impregnated sleeve of a general nature similar to that commonly used at present in magnetron cathode practice. This sleeve is referenced 13. In accordance with this invention, however, the sleeve is provided with grooves 12 equiangularly spaced around it and running parallel to its axis. In these grooves are positioned lengths of filament 11 which are tensioned by means not shown between end connector rings 9. The filaments 11 must be insulated from the sleeve 13 but since they are straight filaments it is not difficult by suitable design to secure the required insulation simply by tensioning the filament lengths 11 to hold them clear of the walls of the grooves in which they run. In the case of a long cathode structure it may be advisable to provide a few spaced insulating beads through which the filament lengths are threaded or, as before, layers of insulation may be provided in the grooves on the body 13. As in FIGURES 1 and 2, 4 represents part of the cathode support structure and 8 are the end hats. In FIGURE 4 the left hand connector 9 of FIGURE 3 is removed so as not to conceal the grooves 12 and filaments 11.

The barium aluminate impregnated sleeve structure of FIGURES 3 and 4 is so designed that, when the magnetron is in normal operation, it operates at a temperature at present customary in magnetron cathode practice i.e., at a temperature in the neighbourhood of 950 to 1000 C.

In both the illustrated structures the filamentary primary electron emitter, which may be made of any suitable known suitable material (for example tungsten) is of relatively small area and mass and is so arranged that, when switched on, it will rapidly reach a required high temperature. It has quite small thermal inertia and accordingly will rapidly provide the thermionic emission necessary for starting a cold magnetron. Because of its small area it will not appreciably limit the handling capacity of the cathode structure as a whole as respects back bombardment. Accordingly the desired advantage (as compared with a known comparable cathode structure) of rapid warm up without appreciable limitation of back bombardment power handling capacity is obtained. It is expected, moreover, that cathode structures as herein described will have superior long term secondary emission properties.

It will be observed that the characteristic feature distinguishing the present invention from usual known practice resides in the substantial separation of the functions of primary and secondary emission. Although as above stated, the non-filamentary part of the cathode structure may in practice contribute a certain amount of primary emission when the magnetron is in operation, this does not happen until after an initial starting period is over and, from the practical point of view, the filament may be fairly regarded as providing primary emission for quick starting and the secondary emissive cathode mememission, the electrons required to maintain the mag netron in normal operation.

The design may be and preferably is such that, in the case of a cathode surface providing both primary and secondary emissione.g. a barium aluminate impregnated surfacethe filamentary primary emitter may be switched off when normal operating conditions have been reached.

I claim:

1. A magnetron having a cathode structure comprising a secondary electron emissive cathode member adapted to provide at least the major part of the required electron emission by secondary emission by electron bombardment when the magnetron is in normal operation, and a filamentary thermionic emitter adapted to provide emission for rapidly starting said magnetron into operation from cold.

2. A magnetron as claimed in claim 1 wherein the secondary emissive cathode member is an externally grooved body having an outer surface of high secondary electron emission coefficient and the filamentary thermionic emitter is constituted by filament wire positioned in the grooving.

3. A magnetron as claimed in claim 2 wherein the secondary emissive cathode member is helically grooved on its external surafce and thermionic emissive filamentary wire is positioned in the helical grooving.

4. A magnetron as claimed in claim 2 wherein the secondary emissive cathode member is formed with grooves distributed round it and running parallel to its axis and thermionic emissive filamentary wires are positioned in these grooves.

5. A magnetron as claimed in claim 1 wherein the secondary emissive cathode member is a massive solid body of beryllium alloy having an oxidised surface adapted to provide the required secondary emission at a relatively low temperature, said body being kept down to said low temperature, when the magnetron is in normal operation, with the aid of loss of heat by substantial heat conduction occurring through said body.

6. A magnetron as claimed in claim 1 wherein the secondary emissive cathode member is a barium aluminate impregnated sleeve adapted to provide the required primary and secondary emission at a temperature in the neighbourhood of 950 to 1000 C. when the magnetron is in normal operation.

7. A magnetron as claimed in claim 1 wherein the filamentary thermionic emitter is a wire which is threaded through insulating beads which are spaced along it and space it from the secondary electron emissive cathode member.

References Cited UNITED STATES PATENTS 2,097,302 10/1937 Passarge 313337 X FOREIGN PATENTS 940,181 3/ 1956 Germany.

JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner. 

1. A MAGNETRON HAVING A CATHODE STRUCTURE COMPRISING A SECONDARY ELECTRON EMISSIVE CATHODE MEMBER ADAPTED TO PROVIDE AT LEAST THE MAJOR PART OF THE REQUIRED ELECTRON EMISSION BY SECONDARY EMISSION BY ELECTRON BOMBARDMENT WHEN THE MAGNETRON IS IN NORNAL OPERATION, AND A FILAMENTARY THERMIONIC EMITTER ADAPTED TO PROVIDE EMISSION FOR RAPIDLY STARTING AND MAGNETRON INTO OPERATION FROM COLD. 